Your are boosting the contrast by this S-shaped curve.
The midtones are stretched over a wider range in the histogram. By pulling down the curve on the left side dark tones getting darker (left hill in the histogram after). On the right side light tones are pushed a little bit to get lighter (right hill in the histogram).
There is a nice thread here about histograms and tone curves were you can find much better explanations what is going on:
You can set the blend mode to difference to see what parts of the image are effected by the module and the changes.
Watching the last video I noticed the halos this tool could create. It looks like what shadows and highlights module in dt produces if pushed too far with the Gauß filter. So I guess there is some bluring filter around the tonal range controlled by the sliders to avoid harsh tonal breaks. Sorry, for this silly describtion, but I have no idea about the correct terms.